Cathode ray deflecting electrode



June 6, 1939. c. s. SPRAGUE ET AL 2,161,437

CATHODE RAY DEFLECTING ELECTRODE Filed Sept. 50, 1935 INVENTORS Patented June 6, 1939 GATHODE RAY DEFLEOTINGELECTRODE Carlton S. Sprague and Roscoe H. George, West -Lafayette, Ind., assignors, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application September 30, 1935, Serial No. 42,823 Claims. (01. 250-159) N This invention relates toelectrode structures and, in particular, to electrostatic deflecting .elec' trodes such, for example, as may be used in cathode ray apparatus for deflecting a'beamof elec- 5 trons.

" The invention is concerned with overcoming some of the difliculties hitherto experienced with electron beam deflection system, such as keystoning and spurious velocity modulation. If in one type of a cathode raytube, for example, a beam of electrons is developed and directed to impinge upon a fluorescent'endwall and to traverse two mutually perpendicular electrostatic fields which are also perpendicular to the beam,

l5gthen with suitable variation of the electrostatic field, such as a sinusoidal variation, a rectangular area of illuminationvshould appear upon the tube endwall. However, it will be found that instead of a rectangular pattern, the actual produced pattern will closely approximate a trapezoid, due to, in part, the stray flux of the deflecting plate system producing distortion... The close resemblance of the producedtrapezoidal area to the section of a keystone is responsible. for this type I 25 of distortion being called keystone effect or more simply keystoning.

In the usual construction of deflecting plate structures it is impossible to produce electrostatic- 1 fields without stray flux, and consequently, the

30 field between deflecting plates is not uniform over the entire area of the plates. At the edges of theplates, particularly, the electrostatic lines of force exhibit large and varying curvature resulting in a non-uniform field. In the central portion of the deflecting plates, however, the lines of force are essentially parallel to each other and perpendicular to both, of the parallel plates, so that a substantially uniform field results. sequently when a beam of electrons traverses the space between the deflecting plates, deflecting mutually perpendicular plates for deflecting the H beam of electrons requires that each pair of plates shall act upon the beam in only one direction, and the motion; under these conditions will be pera, pendicular to the plates viewed from above. This is well known in the art, and for further reference the Nicolson Patent No. 1,479,696 may be consulted as well as the Johnson Patent No.

1,565,855. Obviously the production of an electrostatic field parallel to the plates will produce 5 motion parallel to the plates and consequently introduce distortion since the motion parallel to one pair of plates should result only from the flux produced by the other set of plates. It will therefore be seen that it is extremely advanta- 1 geous to eliminate the spurious motion of the electron beam produced by the non-uniform field at the edges of the plate.

The distorted electro-static field acting along the path of beam of electrons produces a still further distortion, inasmuch as the component of the field acting along the path of the electron beam tends to accelerate or decelerate the beam depending upon the sign of the field. Since the magnitude of this acceleration or deceleration is a function of the voltage applied to the plates, it will be readily appreciated that the beam velocity will be modulated'in accordance with the applied voltages.

This is undesirable, since modulating the beam velocity results in the intensity of illumination of the fluorescent end-wall being non-uniform. At the same time, the variation beam velocity produces the undesirable feature of variable beam deflection sensitivity, since, as is well known, in-

creasing the beam velocity reduces the produced deflection of the beam when the deflecting field is held constant. The deflection increases as the beam velocity is reduced under this same condi tion of constant field. Obviously the elimination V of non-uniform electro-static field intensity at the edges of the deflection plates will enhance, in a large degree, the operation of cathode ray tubes, for example. I

Further, than this, the increasing use of cathode ray tubes for oscilloscopic purposes in industry as a measuring instrument requires that the deflection of the cathode ray be substantially proportional to the applied deflecting voltages over the entire viewing or recording area. t will be appreciated that theelimination of the uncontrolled stray electrostatic field from the edges of the deflecting electrodes will permit cathode ray tubes to function so as to fulfill this desired condition. I

Our invention, which is directed toward the elimination of these uncontrolled stray electrothe application of shield electrodes adjacent to tially predetermined in space. These shield elec'- trodes control the stray flux and produce thereby substantially uniform electrostatic fields through which the beam of electrons passes. Consequently keystoning and velocity modulation are substantially eliminated.

By the use of the shield electrodes, the-com' ponent of the electrostatic field parallel to the 20 cathode beam is restricted to the gap between the shield and deflecting electrodes and thus is ineffective as regards the influencing of the velocity of the beam of electrons. The fringing of the electrostatic field in the 'gap between'the' shield and the deflecting electrodes is substantially without influence on the beam of electrons since the leakage-flux is small and confined to a very small volume immediately adjacent to the gap, as will be readily appreciatedfromtheconcfigura'tion of the electrodes.

For a more complete description and understanding of this invention refer now to the drawing of which Figure 1 shows one embodiment of the inven- Figure 2 shows a view which is a section through 22. I

Figure 3 is a section through "33, 'and Figure 4 shows a modification of the invention.

Referring now to Figure 1, the tube wall I is shown supporting a pair of deflecting electrodes 3 through the medium of the insulating supports 5. Mounted on the deflecting electrodes 3 are insulatirg support members I, which support di 45 rectly above and below and in the same plane as the electrodes 3, strips of metal 9. These strips ofmetal are connected by lightflexible' leadsll 7 to the wall I, which in Figure 1 is assumed to be made of metal.

50 the shield electrodes 9 and the electrodes 3. The

disposition of the component parts of this struc- .ture is shown in more detail in the section draw-- ings of Figure 2 and Figure-*3 where like parts carry the same reference numeral.

5 5 It is to be understood that while the 'drawin construction, of the cathode ray apparatus; the modiflcationshown in- Figure 4 may be used ad 1 c5'- t v A metal strip is bent into an oval shape and spot welded to form 'a flattened hoop ll asshown in Figure 4. The strip maybe of any metal but it is desirable that a hard non-porousnon magneticmetal such as tantalum be used, in order' to facilitate degassing of the metallic parts in "the latter stages of processing the *tube. In order to avoid spurious "magnetic deflectingfields; non magnetic metal shouldbe used for the shield and A small gap I3 is left between may be of the order of .006 inch although the thickness is not critical. However, from the standpoint of ease of working and economical construction, this value has been found to be satisfactory. 5

It has been found that the width of the electrode l'l determines the effectiveness of shielding and that the optimum width depends on the spacing of the deflecting electrodes. Good shielding is obtained provided the ratio of the spacing of 10 the deflecting electrodes 3 tothe width of the shield electrodes I! does not exceed 3. These hoop-like shield electrodes I! are spot welded to short supporting 'members I9, which for convenience may be No. 20 B, & S. gage tantalum wire, which are also spot welded to one of the two defleeting electrodes 3, which is the one usually grounded. The shield electrodes I! are so positioned that a small gap gene-rally smaller than the width of electrode l1 exists between it and the edge of 'the deflecting electrodes 3 adjacent to the shield I1. The use of this construction enables the stray electrostatic field to be controlled and directed to'the shield electrodes II. By making the length of the electrode I! slightly larger than the width of the electrodes 3, stray flux from the edges of the electrodes 3 will converge on the electrodes I? rather than impinge .on exterior or other adjacent conductors. Obviously the length of theelectrode I! should not be less than the width of the electrode 3 in order to prevent distorting the electrostatic deflecting field produced within the space separating the two electrodes 3. It has been found that a satisfactory length of the shield electrode 11 is obtained when its length is equal to the sum of the width of the electrodes 3 and twice the distance between them. An electrostatic field is set up in the gaps 2| which is in the direction of the path of the cathode ray beam. But as explained above in trolled components ofithe stray ele'ctrostatioflelds accomplished through the use of the shield electrodes, l1 eliminates keystoning, as, explained above.

It will be appreciated that while shield electrodes might be applied to the vertical edges of 5 the deflecting electrodes3, in practice this is not generally necessary inasmuch as the beam is not deflected sufliciently toward these edges to be under the influence 'of'the distorted electrostatic field along these 'edges. 23-;

pair of elongated spaced andparallelly'disposed deflecting electrodes; a ring likeelectrode positionedatj at least-one end of said deflecting electrodes said'ring-like*electrode'lying in a plane parallel to the direction of'inovernent of the beam between the deflecting plates',.said ring-like elec- 2. In a cathodera'y tube-deflecting electrode 5? assembly, a pair of parallel spaced deflecting electrodes, a shielding electrode bounding the end plane of each of said deflecting electrodes, supporting means for the deflecting electrodes, means for supporting the shielding electrodes in prede- 1 termined spaced relation relative to the deflecting electrodes, and connecting means to establish electrical contact between one of the deflecting electrodes and the adjacent shielding electrode.

3. In a cathode ray tube, in combination, a pair of electrostatic deflecting electrodes, shielding electrodes supported thereon each end thereof and insulated therefrom, said shielding electrodes each having a surface thereof co-planar with the edge of the adjacent deflecting electrode and connecting means for establishing electrical contact between said shielding electrodes and one of said deflecting electrodes.

4. In an electron tube wherein there is developed a controlled electron stream, a pair of opposed electrodes positioned adjacent to the normal path of the developed electron. stream for deflecting the electron stream when suitable energizing potentials are applied thereto, at least one shielding electrode positioned to bound at least one end of the pair of opposed electrodes, said shielding electrode being spaced apart from said deflectingelectrodes and positioned also to surround the normal path of the electron stream and to have a surface thereof co-planar with each deflecting electrode, and an electrical connection between said shield electrode and at least one of the opposed electrode members.

5. In a cathode ray electron beam deflecting system a pair of parallel spaced deflecting electrodes, a shielding electrode comprising a tantalum member surrounding a plane equal in length to the width of each electrode and equal in width to the spacing between said electrodes, supporting means for the deflecting electrodes, means for supporting the shielding electrodes in predetermined spaced relation relative to the deflecting electrodes and co-planar therewith said spacing being smaller than the width of said shielding electrode, and connecting means for establishing electrical contact between one of the deflecting electrodes and the shielding electrodes.

CARLTON S. SPRAGUE. ROSCOE H. GEORGE. 

